Pressure measurements made before, after, or during a therapeutic or diagnostic procedure can be important methods of analyzing any body conduit. In blood vessels, pressure measurement may be used to continuously monitor a patient's condition, to determine the patency of a specific artery or vessel, to assess the severity of a lesion or stenosis, or to asses the results of a therapeutic procedure such as angioplasty, atherectomy, or stenting. Pressure measurements may be of two types, phasic or flat line. The flat line pressure is the average of pressure changes over time. The phasic pressure is a wave form. While the average pressure is of interest, physicians can readily identify a true physiologic wave form, with frequency content from DC up to about 30 Hz, and compare it to expected norms, thereby making phasic pressure measurement a highly valuable diagnostic tool.
Percutaneous coronary angioplasty is a specific procedure in which pressure measurement may be a valuable tool for lesion assessment and therapy assessment. The catheter which is used to measure pressure must be small enough so that the catheter itself does not interfere with measurement. In the epicardial coronary arteries, this requires catheters which are a fraction of a millimeter in diameter. It is also preferred to make the pressure measurement from a catheter which is already being used in a procedure, rather than exchanging for a pressure measuring catheter.
Prior art devices disclosed by Hastings, et al. in U.S. Pat. No. 5,450,853, Wise, et al. in U.S. Pat. No. 5,113,868, and Little in U.S. Pat. No. 5,313,957 have integrated micro-sensors into the distal end of a guide wire with an electrical or optical interconnect extending to the proximal end of the wire (approximately 1.8 meters). Since the wire is only 0.014 inches in outer diameter, it is very difficult to integrate the sensor and interconnect into the guide wire without altering the mechanical performance of the wire. The wire must torque, push, and steer sufficiently well to navigate the tortuous coronary vasculature. Wires with integrated distal sensors which accomplish this feat are inherently expensive to produce.
Prior art fluid lines which provide a phasic pressure signal are typically underdamped and have a diameter much larger than a guide wire. As an example, Model PXMK099 from the Edwards Critical Care division of Baxter Health Care in Irvine, Calif. consists of a pressure transducer with a six inch connecting pressure tube connected to a user supplied fluid filled tube. When the Baxter system is connected to a 0.014 inch hollow guide wire, the output signal is totally damped and only a flat line average pressure is displayed. This damping is due to the relatively high compliance of the Baxter system and the relatively large volume of water contained therein. To determine the minimum tube diameter which can transmit a phasic blood pressure signal through the Baxter system, 1.8 m long polyimide tubes of varying diameters ranging from 0.012-0.057 inches were connected via a Touey-Borst style connector to the Baxter system. Experiments on this system found an average system compliance of ##EQU1## and that the natural frequency was greater than or equal to 30 Hz in lines with diameters greater than 0.053" (0.0013 m). The lines with diameters less than 0.020 inches were over-damped and the lines with diameters larger than 0.020 inches were under-damped. Clearly prior art fluid lines which provide adequate frequency response are much larger than guide wires and still are not critically damped.
Another prior art device is disclosed by Tremulis in U.S. Pat. No. 4,953,553. Tremulis discloses a small diameter fluid filled line which can be used as a guide wire. However, blood pressure signals from this device may be extremely damped, giving only an average pressure value.
Therefore, it would be advantageous to provide a medical pressure sensing device with the reduced cost attributes of a fluid line, a small enough diameter to be used as a guide wire or to be used in small vessels, and sufficiently responsive to provide a phasic pressure signal.